Superalloys are metallic materials, usually based on nickel or cobalt, which have especially useful properties at temperatures of about 1,400.degree. F. and above. Nickel base superalloys derive much of their strength from the presence of a strengthening phase precipitate typically referred to as gamma prime Ni.sub.3 (Al, Ti); the amount of and morphology of the gamma prime phase strongly affects the mechanical properties of these materials. Gamma prime precipitates may be solutioned into the alloy matrix when heated above the solvus temperature.
Superalloy articles sometimes also contain as-cast, segregated phases which melt at a temperature which is below the liquidus temperature of the article. Such low temperature melting is called incipient melting, and its presence in a casting can compromise the mechanical properties of the casting. The fact that the incipient melting temperature is sometimes in the same range as the gamma prime solvus temperature complicates the heat treatment of such alloys.
Heat treatments for various superalloys are described in, e.g., U.S. Pat. Nos. 2,798,827, 3,310,440, 3,753,790, 3,783,032, 4,209,348, 4,116,723, and 4,583,608. Several of these patents teach that the incipient melting temperature of nickel base superalloy castings may be increased by slowly heating the casting to a temperature just below its incipient melting point. Such heating causes some of the segregate phases to diffuse into the alloy matrix, thereby increasing the casting's incipient melting point. The temperature of the article may then be further increased, which allows for more diffusion of segregate phases into the matrix, and a further increase in the incipient melting point. U.S. Pat. Nos. 3,753,790 and 3,783,032 and U.S. Ser. No. 501,662 describe one such heat treatment for nickel base superalloy castings, wherein the article is heated to a first temperature and held at that temperature to permit diffusion of the segregate phases, and then heated and held in a stepwise fashion to a series of higher temperatures, as shown in FIG. 1. Alternative heat treatment cycles are described in the U.S. Pat. Nos. 3,753,790 and 3,783,032 patents: after the initial hold at the first temperature, the castings are heated at a gradual but continuous (constant) rate to a maximum temperature T.sub.max to further diffuse the segregate phases into the matrix. Such a heat treatment cycle is shown in FIG. 2.
Both the step temperature and constant rate heat treatment cycles of the prior art are lengthy; since the cost of a heat treatment increases with time, engineers have sought improved cycles to produce castings with optimum properties, wherein the heat treatment time is minimized.